The purpose of these studies is to quantitate changes in regional and organ vascular resistance in the development and treatment of hypertension. Past studies have shown that benign hypertension is often characterized by normal values for total cardiac output and increased values for total peripheral resistance. But, we specifically want to determine: 1) If there is excessive vasoconstriction and hypoperfusion in selected organs or regions in severe hypertension. 2) If successful antihypertensive therapy lowers arterial pressure via homogeneous vasodilation or if certain organs are in fact not benefited (or harmed) by the pressure decrease. 3) If there is a consistent regional vasoconstriction in hypertensions of different etiologies (one-kidney Goldblatt, two-kidney Goldblatt, Okamoto spontaneous, and desoxycorterosterone plus salt) that suggests the presence of a final common vasoconstrictor mechanism. Microsphere techniques will be used in conscious rats. Quantitation of regional and organ resistances in a physiological setting requires that arterial pressure, cardiac output and flow distribution all be known in each animal studied and that the undesirable influences of anesthesia be avoided. We have developed methods for use in conscious rats in which fifteen micron microspheres are injected through a chronically placed left atrial catheter and cardiac output is measured by the reference organ technique. Many previous studies have lacked one or more of the requisite values and consequently our understanding of the control of vascular resistance and organ perfusion in benign hypertension, severe hypertension, and during therapy remains incomplete. Our present methods will provide numerical values that will allow rigorous analysis of regional perfusion and resistance changes in the spectrum of circumstances described above.